Numerical experiments on the residence time distributions of heterogeneous groundwatersheds

Groundwater residence time distributions are important when studying the impacts of non-point source poolution in shallow groundwatersheds (aquifer-stream systems) on stream water quality. The theoretical residence time distribution F(T) for unstratified groundwatersheds that are homogeneous with respect to porosity, recharge and aquifer thickness is a simple exponential function. This function F(T) is insensitive to watershed size, stream network and spatial distribution of hydraulic conductivity. It has been found that certain of the conditions previously presented as necessary for the validity of F(T) can be somewhat relaxed. In this paper, we explore the behavior of residence time distributions when these assumptions used in the development of F(T) are no longer met, and test the robustness of the theoretical distribution F(T) in such cases. Numerical experiments suggest that F(T) provides a suitable approximation to many groundwatersheds' residence time distributions, except when aquifer heterogeneities are few and distinct. Additionally, a new residence time distribution (F) over cap(T) is derived for a class of semi-confined stratified aquifers. (C) 1998 Elsevier Science B.V. All rights reserved.